U.S. Pat. No. 4,032,143 issued June 28, 1977, describes a composite molded baseball bat having a tubular, high yield aluminum core surrounded by a foam plastic body, preferably a polyurethane body, having a density of from 30-50 pounds per cubic foot. This patent describes a method for producing a baseball bat in which the tubular aluminum core is mounted vertically in a mold and the foam plastic is poured into the mold to surround the tube and is then allowed to solidify. This results in a bat lacking a knob at the handle end and a plastic knob is subsequently glued or molded on to close or cap the tube at the gripping end of the bat.
One of the problems encountered when using the method of the above patent is whether the plastic body, e.g. polyurethane, may fail to adhere adequately to the tube resulting in an actual slippage of the tube relative to the plastic body. It is believed that such lack of adhesion may be caused by the surface oxidation of the aluminum. In any event, such lack of adhesion would result in a bat which is not suitable for its intended purpose. On the other hand, should the plastic body adhere to the tube, shrinkage of the plastic body on curing tends to be resisted by compression in the tube, which thus imposes stresses on the plastic. Therefore, the plastic body may crack or fracture, either on curing or, more usually, after some period of use, the cracks extending radially outward from the tube to the outer skin of the plastic body, thus rendering the patent unfit for use. It is believed that compression in the tube, induced by the shrinkage of plastic adhering to it, opposes the shrinkage of the plastic body during curing, and the resulting nonuniformity of shrinking produces tension or stress zones, or areas which ultimately are relieved by cracking, as when the bat strikes a baseball during use.
U.S. Pat. No. 3,971,837 issued July 27, 1976, describes a composite foamed bowling pin having a steel or aluminum pipe or tube surrounded by a cellular polyurethane body which is in turn surrounded by a solid sheet of thermoplastic resin, e.g. cellulose acetate, to provide an exterior finish for the bowling pin. To the extent that this method utilizes an aluminum tube or pipe around which the polyurethane body is molded, the same problems are encountered as are referred to above in regard to U.S. Pat. No. 4,032,143.